1. Field of the Invention
The present invention relates to a micro electro-mechanical system (MEMS) gyroscope for measuring rotational angular velocity of various devices. More particularly, the present invention relates to a vertical MEMS gyroscope by horizontal driving in which motions of a driving mass and detection mass are performed independently of each other.
2. Description of the Related Art
Gyroscopes, well known for rotational angular velocity detection, have been widely used in many areas, and are particularly important as a core part of navigation equipment in ships and airplanes. Nowadays, the development of the gyroscope has enabled application of the same in areas such as navigation systems for automobiles and hand-tremor compensation devices for high-resolution video cameras.
Gyroscope operation is based on the Coriolis force. When there is a certain mass vibration in a certain direction, say, in a direction of a first axis, and a rotational force of a certain angular velocity is applied at a right angle to the mass vibration, i.e., in a direction of a second axis, a Coriolis force is generated in a direction of a third axis, which is at a right angle to the first and second axes.
In order to generate and detect the Coriolis force, the gyroscope is provided with a mass and a detection electrode, which vibrate in a certain direction. Hereinbelow, the direction in which the mass of the gyroscope vibrates will be called a ‘driving direction’, and a direction in which rotational angular velocity is input with respect to the gyroscope will be called an ‘input direction’. Also, a direction in which a Coriolis force of the mass is detected will be called a ‘detecting direction’.
In space, the driving direction, the input direction and the detecting direction are set at right angles to each other. Usually, in the MEMS gyroscope, three coordinate axes are set, including two directions which are parallel to a plane of a substrate and perpendicular to each other (hereinafter called a ‘horizontal direction’), and one direction at a right angle with the plane of the substrate (hereinafter called a ‘vertical direction’).
Gyroscopes are usually divided into two types, i.e., a horizontal type (Z-axis) and a vertical type (X- or Y-axis). A horizontal gyroscope has horizontal driving and detecting directions and a vertical (Z-axis) input direction, while a vertical gyroscope has a horizontal (X or Y axis) input direction.
In a conventional horizontal type gyroscope that uses a silicon on insulator (SOI) structure, an angular velocity that is input about a Z-axis, which is perpendicular to a plane of a substrate, can be measured, but an angular velocity in two axes on a same plane cannot be measured. Accordingly, in order to measure multiple-axis angular velocity, a process of arranging an element vertically is additionally required, which causes a considerable increase in cost, as well as degradation in reliability and performance.
It has been suggested to detect rotational angular velocity in a horizontal direction using a vertical MEMS gyroscope. In order to detect the angular velocity input in the horizontal direction (X- or Y-axis), a driving electrode that drives a mass vertically or a detection electrode that detects vertical displacement of the mass is required.
A conventional way to fabricate a vertical driving or detection electrode is to form a fixed electrode on a substrate and a motion electrode at a predetermined distance upwardly from the fixed electrode. When the fixed electrode and motion electrode are used as a driving electrode, a variable voltage is applied between the motion electrode and the fixed electrode to drive the motion electrode. When the fixed electrode and motion electrode are used as a detection electrode, an electrostatic force varying in accordance with the distance between the fixed electrode and the motion electrode is detected. Therefore, the angular velocity is measured.
However, a structure in which a motion electrode is formed upwardly from a fixed electrode has a shortcoming in terms of a complicated fabricating process. In order to fabricate the electrodes, first, the fixed electrode is fixed on the substrate and a sacrificial layer is deposited on the fixed electrode. Then the motion electrode is formed on the sacrificial layer and the sacrificial layer is removed.
Furthermore, the distance between the motion electrode and the fixed electrode should be small in order to precisely measure displacement of the motion electrode in the vertical direction. However, this may cause an adherence of the motion electrode and the fixed electrode.
FIG. 1A illustrates a sectional view of a horizontal MEMS gyroscope, and FIG. 1B illustrates a sectional view of a vertical MEMS gyroscope by vertical driving. Referring to FIGS. 1A and 1B, there is a support layer 110, 210 on a substrate 100, 200, and a MEMS structure 500, 600 is formed on the support layer 110, 210. There is a cap wafer 150, 250, which is attached to an upper portion of the MEMS structure 500, 600. The cap wafer 150, 250 is attached to the upper portion of the MEMS structure 500, 600 in a vacuum chamber. In order to secure sufficient space to maintain a vacuum inside the gyroscope, there is a predetermined space 160, 260 defined at a lower portion of the cap wafer 150, 250.
The MEMS structure 500, 600 includes a driving structure 120, 220, a detecting structure 130, 230, and an electrode anchor 140, 240. The MEMS structure 500, 600 is floating above the substrate 100, 200 with a part thereof being fixed to the support layer 110, 210. Albeit not shown in FIGS. 1A and 1B, the driving structure 120, 220 includes a driving mass, which vibrates in a predetermined direction, a driving electrode, which drives the driving mass and a detection electrode, which detects any displacement that occurs by the vibration of the driving mass. The detecting structure 130, 230 includes a detection mass, which vibrates in a predetermined direction, and a detection electrode which detects any displacement of the detection mass. Electric signals from respective detection electrodes are output to an external electrode 180, 280 through the electrode anchor 140, 240 and a via hole 170, 270 formed through the external electrode 180, 280.
In the case of a horizontal MEMS gyroscope, the respective detection electrodes for detecting the displacement of the driving mass and the detection mass are on a same plane as the driving mass and the detection mass, and respective motion electrodes vibrate in the horizontal direction together with the corresponding masses.
However, in the case of a horizontal MEMS gyroscope by vertical driving, the structure of the driving electrode for driving the driving mass in the vertical direction with respect to the substrate, and the structure of the detection electrode for detecting the displacement of the driving mass are different from those of the detection electrode of the detection mass. In a driving electrode and a detection electrode of comb-structure that are formed on a side of the driving mass, a motion electrode vibrating in a vertical direction should be shorter than a vertical length of a fixed electrode. Accordingly, a fabrication process becomes complicated, and a vertical MEMS gyroscope cannot be fabricated together with a horizontal MEMS gyroscope. Further, when the driving mass is driven vertically by the driving electrode formed at the side of the driving mass, the displacement of the driving mass is non-linear.